Lawmakers concerned by rockets "turned over to local authorities" by the UN organization.

WASHINGTON — Members of the United States Senate are demanding an independent investigation into the role of the United Nations Relief and Works Agency during Israel's most recent war in Gaza with Hamas.

Accusing UNRWA of maintaining active and extensive ties with Hamas— and of supporting its activities throughout the month-long war— Senate Foreign Relations Committee members Mark Kirk (R-Ill.), Ben Cardin (D-Md.) and Marco Rubio (R-Fla.) wrote a letter this week to US Secretary of State John Kerry accusing the UN agency of bias and characterizing its role in the conflict as "troubling."

UNRWA, an ostensibly neutral agency tasked with administering aid to Palestinian refugees throughout the region, adopted a political role in the heat of the conflict, during which at least four of its facilities were badly damaged and many of their inhabitants killed. During the deadliest days of the war, UNRWA officials went on record accusing the Israeli government of violating international humanitarian law.

UNRWA also publicly declared the discovery of three caches of rockets stored in Gaza schools during the July battle. The organization did not identify a responsible party for the crime, however, noting that the schools used as weapons depots were "mothballed" for the summer months.

Media reports quickly surfaced suggesting UNRWA returned the recovered rockets to Hamas, but those claims were never independently unverified.

"UNRWA claimed to have turned over [the rockets] to the 'local authorities' or have gone missing," the Senate letter reads. "We fear that this means these rockets may have found their way back into Hamas' hands."

The senators note that the US government is the single largest donor to UNRWA, providing the agency with $294 million in 2013 and a total of $5 billion since 1950.

While the letter does not call on the State Department to cut aid, the senators write that the American taxpayers "deserve to know if UNRWA is fulfilling its mission or taking sides in this tragic conflict."

The United States and European Union list Hamas as a terrorist organization, and the United Nations has called on the group to renounce violence, recognize Israel, and respect previous agreements between the Palestinian Authority and the Jewish state.

Responding to the letter, a State Department spokesman said that the UN is taking "proactive steps to address this problem," including deploying munitions experts to the strip in search of more weapons caches.

"The international community cannot accept a situation where the United Nations– its facilities, staff and those it is protecting– are used as shields for militants and terrorist groups," State Department spokesman Edgar Vasquez told The Jerusalem Post. "We remain in intensive consultations with UN leadership about the UN’s response."

Hamas' use of UN facilities as "shields" for its fighters and its weapons posed one of the most politically vexing challenges of the war. UN secretary-general Ban Ki-moon said that such crimes turned UN facilities into legitimate military targets, inviting Israeli strikes; but in a strongly-worded statement from the State Department, after an Israeli shelling killed UNRWA refugees in Gaza for a third time, the US said "the suspicion that militants are operating nearby does not justify strikes that put at risk the lives of so many innocent civilians."

The Israeli army fiercely denies that it targets civilians, arguing that its use of leaflets, phone calls, text messages and "knocks on the roof" warning of impending strikes are indicative of its efforts to avoid civilian casualties.

"There are few good solutions given the exceptionally difficult situation in Gaza," Vasquez continued, "but nonetheless we are in contact with the United Nations, other UNRWA donors and concerned parties— including Israel— on identifying better options for protecting the neutrality of UN facilities and ensuring that weapons discovered are handled appropriately and do not find their way back to Hamas or other terrorist groups."

Kirk, one of the signatories of the letter, said that UNRWA has had "ties to terrorism" in the past, and that, in September 2012, Hamas–affiliated candidates won 25 out of 27 seats on UNRWA's workers union board.

"I am demanding a credible and independent assessment of UNRWA's actions during this crisis," Kirk said in a statement. "US taxpayers deserve immediate answers and full transparency regarding their intentions and actions."

Cardin was the sole Democrat among the three behind the letter.

“When leaders and organizations of the United Nations blur the clear distinction between a nation-state defending itself and a terrorist organization attempting to murder civilians, Americans take note," Cardin said. "When an organization funded in part by the US suggests that the two are morally equivalent, US taxpayers take note."